Can the perpetuation of polymorphism in Cepaea be as simple as black or white?
Polymorphisms are allelic differences at given loci within a population; they are the raw materials for evolution. Novel alleles are caused by genetic mutations, these are then either maintained or lost via several processes. Allele frequencies within populations can be affected by selection for or against specific genotypes. Also, genetic drift makes stochastic changes to heterogeneity at a locus, this may increase differences at times, but it ultimately removes them by fixation. Polymorphisms may also be introduced into a population by gene flow from another population, however gene flow can act as a homogenising force too, reducing polymorphism dependent on allele frequencies in adjacent populations.
Polymorphism is found within the snail genus Cepaea, these snails have been studied in the earliest investigations into the genetics of natural populations (Diver, 1929) and the wealth of information collected can only be rivalled by that of information about human populations, (Jones et. al., 1977). Cepaea are numerous and diverse, importantly the polymorphism is extensive and easy to see, there are few monomorphic colonies in Britain. Helpfully the major differences in their genetics understood, the polymorphism being largely the product of just one supergene, (Jones et. al., 1977). Their low dispersal of only a few metres per year and historically stable populations (Slatkin, 1985) enable potential factors of polymorphism maintenance to be easily studied. Theoretical models derived from studying organisms such as Cepaea can be applied more generally to many other species, including humans.
In this investigation evidence for the existence and causes of polymorphism in C. nemoralis populations living in a heterogeneous environment will be sought. Morph frequency in two adjacent habitats, grassland and moderately dense scrub, will be sampled to test two hypotheses:
1. Natural selection exerts a stronger influence on morph frequency in each habitat than genetic drift.
2. Gene flow occurs between habitat populations but is attenuated by selection.
To test the first hypothesis, more than one sample will be taken from each habitat-type. If morph frequency data from samples from the same habitat-type are not significantly different, and frequency data between habitat-type samples are significantly different, this may be evidence that selection is taking place. If no correlation is found between morph frequency and habitat-type, drift may be tentatively inferred as a cause of frequency differences. To test the second hypothesis, samples will be taken along transects established between habitats. If a cline in morph frequency is observed along transects, the steepness provides evidence for the extent of gene flow between habitats: the more gradual the cline, the greater the amount of gene flow that can be inferred, as habitat-atypical morphs penetrate further into the other environment and breed. A steep cline may imply restricted gene flow, possibly resulting from strong selective pressure as individuals move into environments to which they are poorly adapted (e.g. greater conspicuousness to predators). Cline gradients may therefore be taken as indicators of the strength of selection in both habitats, thus a coefficient of selection can be calculated, (Bantock & Ratsey, 1980; Haldane, 1948; Malet & Barton, 1989; Malet et. al., 1990).
References:
Bantock, C. R., Ratsey, M., (1980) Natural selection in experimental populations of the landsnail Cepaea nemoralis. Heredity, 44, 37-54. Diver, C., (1929) Fossil records of Mendelian mutants. Nature, 123, 183. Haldane, J. B. S., (1948) The theory of a cline. J. Genet.,48, 277-284. Jones, J. S., Leith, B. H., Rawings, P., (1977) Polymorphism in Cepaea: A problem with too many solutions? Ann. Rev. Ecol. Syst., 8, 109-143. Mallet, J., Barton, N., (1989) Inferences From Clines Stabilized by Frequency-Dependent Selection. Genetics, 122, 967-976. Mallet, J., Barton, N., Lamas, G., Santisteban, J., Muedas, M., Eeley, H., (1990) Estimates of Selection and Gene Flow From Measures of Cline Width and Linkage Disequilibrium in Heliconius Hybrid Zones. Genetics, 124, 921-936. Slatkin, M., (1985) Gene flow in natural populations. Ann. Rev. Ecol. Syst.,16, 393-430.
Polymorphisms are allelic differences at given loci within a population; they are the raw materials for evolution. Novel alleles are caused by genetic mutations, these are then either maintained or lost via several processes. Allele frequencies within populations can be affected by selection for or against specific genotypes. Also, genetic drift makes stochastic changes to heterogeneity at a locus, this may increase differences at times, but it ultimately removes them by fixation. Polymorphisms may also be introduced into a population by gene flow from another population, however gene flow can act as a homogenising force too, reducing polymorphism dependent on allele frequencies in adjacent populations.
Polymorphism is found within the snail genus Cepaea, these snails have been studied in the earliest investigations into the genetics of natural populations (Diver, 1929) and the wealth of information collected can only be rivalled by that of information about human populations, (Jones et. al., 1977). Cepaea are numerous and diverse, importantly the polymorphism is extensive and easy to see, there are few monomorphic colonies in Britain. Helpfully the major differences in their genetics understood, the polymorphism being largely the product of just one supergene, (Jones et. al., 1977). Their low dispersal of only a few metres per year and historically stable populations (Slatkin, 1985) enable potential factors of polymorphism maintenance to be easily studied. Theoretical models derived from studying organisms such as Cepaea can be applied more generally to many other species, including humans.
In this investigation evidence for the existence and causes of polymorphism in C. nemoralis populations living in a heterogeneous environment will be sought. Morph frequency in two adjacent habitats, grassland and moderately dense scrub, will be sampled to test two hypotheses:
1. Natural selection exerts a stronger influence on morph frequency in each habitat than genetic drift.
2. Gene flow occurs between habitat populations but is attenuated by selection.
To test the first hypothesis, more than one sample will be taken from each habitat-type. If morph frequency data from samples from the same habitat-type are not significantly different, and frequency data between habitat-type samples are significantly different, this may be evidence that selection is taking place. If no correlation is found between morph frequency and habitat-type, drift may be tentatively inferred as a cause of frequency differences.
To test the second hypothesis, samples will be taken along transects established between habitats. If a cline in morph frequency is observed along transects, the steepness provides evidence for the extent of gene flow between habitats: the more gradual the cline, the greater the amount of gene flow that can be inferred, as habitat-atypical morphs penetrate further into the other environment and breed. A steep cline may imply restricted gene flow, possibly resulting from strong selective pressure as individuals move into environments to which they are poorly adapted (e.g. greater conspicuousness to predators).
Cline gradients may therefore be taken as indicators of the strength of selection in both habitats, thus a coefficient of selection can be calculated, (Bantock & Ratsey, 1980; Haldane, 1948; Malet & Barton, 1989; Malet et. al., 1990).
References:
Bantock, C. R., Ratsey, M., (1980) Natural selection in experimental populations of the landsnail Cepaea nemoralis. Heredity, 44, 37-54.
Diver, C., (1929) Fossil records of Mendelian mutants. Nature, 123, 183.
Haldane, J. B. S., (1948) The theory of a cline. J. Genet., 48, 277-284.
Jones, J. S., Leith, B. H., Rawings, P., (1977) Polymorphism in Cepaea: A problem with too many solutions? Ann. Rev. Ecol. Syst., 8, 109-143.
Mallet, J., Barton, N., (1989) Inferences From Clines Stabilized by Frequency-Dependent Selection. Genetics, 122, 967-976.
Mallet, J., Barton, N., Lamas, G., Santisteban, J., Muedas, M., Eeley, H., (1990) Estimates of Selection and Gene Flow From Measures of Cline Width and Linkage Disequilibrium in Heliconius Hybrid Zones. Genetics, 124, 921-936.
Slatkin, M., (1985) Gene flow in natural populations. Ann. Rev. Ecol. Syst., 16, 393-430.
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